The present invention concerns a method in a paper machine for the measurement of the air penetrability or permeability of fabrics penetrable by air, in particular of a wire or felt, in which the fabric penetrable by air such as the wire is passed over at least one roll where a positive pressure is formed when the fabric runs at the inlet side of the fabric or roll in an inlet nip formed between the moving fabric and a face of the roll, and a negative pressure is formed at an outlet side of the moving fabric or roll in an outlet nip formed between the fabric and the roll face, these positive and negative pressures depending upon the permeability of the fabric penetrable by air.
The present invention also concerns a device in a paper machine intended for carrying out the method for measuring the air penetrability or permeability of fabrics penetrable by air, in particular of a wire or felt, in which the fabric penetrable by air such as the wire is arranged to run over at least one roll whereby the movement of the fabric produces a positive pressure at an inlet side of the web or fabric in an inlet nip formed between the fabric and a face of the roll, and a negative pressure at an outlet side of the fabric or web in an outlet nip between the fabric and the roll face, these positive and negative pressures depending up on the permeability of the fabric penetrable by air.
Measurement of air penetrability (referred to as "permeability" below) of various fabrics penetrable by air is relevant to, e.g., the case of drying fabrics of a paper machine or filters, e.g., in connection with the monitoring of the condition thereof. Such measurements of permeability must be carried out in an industrial environment on the site. Permeability meters are included, e.g., in the standard equipment of each paper mill, and are used in particular in measurements of the permeability of plastic wires.
Plastic wires tend to become blocked by dust or as a result of mangling phenomenon as the wire mesh becomes smaller, which reduces the permeability thereof. A certain permeability is, however, highly important for the operation of the fabrics, e.g. in the case of the drying wires of a paper machine in view of the operation of the pocket ventilation means. On the basis of the measurements of permeability, it is decided whether, e.g., the wire of a paper machine or the filter fabric in a filter device must be replaced or cleansed.
Measurement of permeability may also be used to monitor or to control operation of various devices, for the cleaning or washing of fabrics. For example, in the case of wires, it is possible to judge from measurements of permeability, the condition of a wire at each particular time, so that the wire can be replaced when necessary before it is broken.
It has been a drawback of all of the prior-art methods and devices in the measurement of the permeability of wires, that it has been necessary to stop the paper machine for the time of the measurement of permeability. This is why the measurement has caused highly considerable costs, for the standstill time of a paper machine is highly expensive.